1. Technical Field
The present invention relates to an antenna device and a wireless device, and more particularly, to a portable wireless device and an antenna device used for the portable wireless device.
2. Description of Related Art
A wireless device, such as a cellular phone, has come into wide use, and the application range thereof has been expanded. For example, a wireless device capable of receiving digital terrestrial television (TV) broadcasting has been developed. A type of an antenna device used for the wireless device has been transferred to a built-in case type having good design and usefulness, and an antenna device for receiving the terrestrial digital TV broadcasting is no exception.
Since the terrestrial digital TV broadcasting for a cellular phone uses a frequency in an UHF band lower than a communication frequency, a wavelength thereof is larger than a size of the general cellular phone. Accordingly, in the antenna device for receiving the terrestrial digital TV broadcasting, only the antenna element of the built-in case type is insufficient as a radiation element and the Printed Circuit Board (PCB) is required to be used as the radiation element. However, in such an antenna device, an antenna current tends to be distributed in one direction on the board. For this reason, a null point of a radiation pattern in the direction of the current easily occurs.
In order to solve the above-mentioned problem, it is disclosed by, for example JP-A-2004-96209, that there is a beam diversity technique using two or more antennas in which directions of a null point of a radiation pattern are different from each other. It is also disclosed by, for examples JP-A-2005-277865 or JP-A-2005-217623, that there is a technique to vary a shape of a radiation pattern using an auxiliary antenna element or an impedance matching element.
JP-A-2004-96209 discloses that chip antennas are provided at two adjacent corners on a board, respectively, resonance currents in diagonal directions excited on the PCB from the chip antennas are made perpendicular to each other, and radiation patterns generated by the resonance currents are complemented with each other to thus form an omnidirectional radiation pattern. (e.g., see pages 2, 5, and 6, and FIG. 1.)
In JP-A-2005-277865, there is disclosed a technique in which a half-wavelength compensation antenna is disposed opposite to a chip antenna, one end of the compensation antenna is switched to be opened or grounded, and thus a shape of a radiation pattern formed by the chip antenna and the compensation antenna is made variable (e.g., see pages 2 and 4 to 6, and FIG. 2).
In JP-A-2005-217623, there is disclosed a technique in which a front end of an antenna element loaded with a variable impedance element therein is grounded on a board, the impedance thereof is varied, and thus a shape of a radiation pattern formed by the antenna element is made variable (e.g., see pages 2, 5, and 6, and FIG. 1).
According to the technique disclosed in JP-A-2004-96209, it is required to secure a diagonal length of the PCB, on which the chip antenna is provided at the corner, by one-fourth wavelength. Accordingly, it has been restricted that the technique is applied to transmission or reception in a frequency band lower than a frequency band for communication in a small-sized wireless device such as a cellular phone.
JP-A-2005-277865 can be applied to only 2.4 GHz band or more frequency band. Accordingly, it has been restricted that the technique is applied to transmission or reception in a frequency band lower than a frequency band for communication in a small-sized wireless device such as a cellular phone. In the technique disclosed in JP-A-2005-217623, a variable reactive element and a control circuit thereof are required. Accordingly, a circuit configuration or a control may be complicated.